1. Field of the Invention
The invention relates to radioactive therapeutic seeds. More particularly, the invention relates to improved radioactive therapeutic seeds for the treatment of oncological and other medical conditions.
2. State of the Art
Radioactive seed therapy is a well known and well accepted medical procedure for the treatment of various oncological and other medical conditions. Seed therapy, also known as brachytherapy typically involves the implantation of fifty to one hundred tiny capsules (seeds) into or around a treatment site. The capsules contain a radioactive isotope which irradiates the treatment site at close range without adversely affecting other parts of the body. Brachytherapy has been used successfully in the treatment of various types of cancers such as prostate cancer. It has also been used to prevent the growth or regrowth of tissues in the treatment of various occlusive diseases such as arteriosclerosis and arthrosclerosis subsequent to balloon angioplasty.
Radioactive therapeutic seeds are carefully designed to possess several important qualities. First, they are relatively small, typically approximately 0.025 inch in diameter and approximately 0.16 inch long so that they may be implanted using minimally invasive instruments and techniques. Second, the radioactive isotope must be enclosed in a biocompatible protective package since the seeds are typically not removed and will remain in the body for many years. Third, each seed preferably includes a radiopaque (e.g. high Z material) marker so that it can be located at the treatment site with the aid of fluoroscopy. Fourth, the protective package and the radiopaque marker preferably do not cast "shadows" in the irradiation pattern of the isotope. Fifth, the isotope should be evenly distributed within the protective package so as to avoid any "hot spots" of radiation.
The state of the art of radioactive therapeutic seeds is substantially disclosed in seven U.S. Pat. No. 5,713,828 to Coniglione for "Hollow-Tube Brachytherapy Device", U.S. Pat. No. 5,405,309 to Carden, Jr. for "X-Ray Emitting Interstitial Implants", U.S. Pat. No. 4,891,165 to Suthanthiran for "Device and Method for Encapsulating Radioactive Materials" and U.S. Pat. No. 4,784,116 to Russell, Jr. et al. for "Capsule for Interstitial Implants", U.S. Pat. No. 4,702,228 to Russell, Jr. et al. for "X-Ray Emitting Interstitial Implants", U.S. Pat. No. 4,323,055 to Kubiatowicz for "Radioactive Iodine Seed", and U.S. Pat. No. 3,351,049 to Lawrence for "Therapeutic Metal Seed Containing within a Radioactive Isotope Disposed on a Carrier and Method of Manufacture".
The Lawrence patent describes many of the essential features of radioactive therapeutic seeds. Lawrence describes radioactive isotopes (I-125, Pd-103, Cs-131, Xe-133, and Yt-169) which emit low energy X-rays and which have relatively short half-lives. When implanted at a treatment site, these isotopes provide sufficient radiotherapy without posing a radiation danger to the medical practitioner(s), people in the vicinity of the patient, or other parts of the patient's body. Lawrence further describes a protective capsule which contains the isotope and prevents it from migrating throughout the body where it might interfere with healthy tissue. The capsule is cylindrical and made of low atomic number biocompatible materials such as stainless steel or titanium which substantially do not absorb X-rays. The isotope is coated on a rod shaped carrier made of similar X-ray transparent (e.g. low Z) material and is placed inside the capsule cylinder. The ends of the capsule cylinder are closed by swaging or spinning and soldering or welding. According to a preferred embodiment, Lawrence places a radiopaque marker inside the seed. In one embodiment, the marker is a wire embedded inside the carrier rod. The wire is made of high atomic number material such as gold or tungsten which absorb X-rays.
In 1980, Kubiatowicz made a minor improvement in the basic Lawrence design by providing that the entire isotope carrier be made of radiopaque material such as silver. Kubiatowicz recognized that since the isotope was carried on the entire outer surface of the carrier, there was no need to make the carrier body X-ray transparent as suggested by Lawrence. The larger radiopaque carrier body described by Kubiatowicz makes the seeds easier to see with X-ray or fluoroscopic examination. Thus, the seeds may be placed more accurately at or around the treatment site.
Several years later, Russell, Jr. et al., in U.S. Pat. Nos. 4,707,228 and 4,784,116, explained that the capsule design of Lawrence and Kubiatowicz produces anisotropic angular radiation distribution. According to Russell, Jr. et al., the shell forming techniques used in the Lawrence-type seeds results in large beads of shell material at the ends of the seeds. These beads substantially shield radiation thereby casting shadows in the irradiation pattern of the isotope. Russell, Jr. et al. proposed a new seed design to solve this problem. In particular, Russell, Jr. et al. proposed a seed having a cylindrical container which is sealed with end caps which have a wall thickness that is substantially the same as the wall thickness of the cylindrical container. The end caps are attached to the cylindrical container by welding or crimping.
An alternate solution to the non-uniform radiation pattern of the Lawrence type seeds was proposed by Suthanthiran in U.S. Pat. No. 4,891,165. Suthanthiran's solution was to form a seed capsule from two interfitting sleeves, each having one open end and one closed end. The thickness of the sleeve side walls and their closed ends is such that when the sleeves are interfit in an overlapping manner, the total side wall thickness of the assembled capsule is approximately equal to the end wall thickness.
Other improvements in radioactive therapeutic seeds are disclosed in U.S. Pat. No. 5,405,309 which concerns a safe isotopically pure Pd-103 seed, and U.S. Pat. No. 5,713,828 which discloses a hollow tube seed which can be implanted with suture material.
Despite the fact that radioactive therapeutic seeds have been in use for over thirty years and despite the several significant improvements made in these seeds, many concerns still exist regarding their design and construction.
While significant attention has been given to the methods by which a cylindrical seed capsule is sealed, it is still difficult to seal such a small cylindrical capsule without adversely affecting the functionality of the seed. Most capsules are sealed at an end using solder which causes a shadow and consequent anisotropic radiation distribution. Radioactive therapeutic seeds are typically deployed by injection through a hollow needle into soft tissue. Upon deployment, it is desirable to position the seeds relatively quickly to minimize exposure of radiation to the physician and to minimize trauma to the patient. It is also desirable to relatively space the seeds according to the prescriptive dose of radiation to be applied at a physiological site and the concentration of the radiation. Moreover, it is desirable to relatively space apart the seeds in a manner which does not affect the distribution of radiation from the seeds.
U.S. Pat. No. 5,460,592 to Langton et al. discloses a plurality of seeds disposed in a bioabsorbable material to facilitate delivery. However, the device has several drawbacks. First, the chain of seeds linked by the bioabsorbable material has very little compression strength. Therefore, it is difficult to feed the seeds together to treatment site. Second, by disposing the seeds within the material, the effective diameter of each seed is increased by the thickness of the bioabsorbable material surrounding the seed. Yet it is desirable to restrict the diameter of each seed to as small as possible to minimize the size of needle inserted into the body and thereby minimize trauma to the patient. Additionally, U.S. Pat. No. 4,784,116 to Russel, Jr. et al. discloses utilizing discrete links to couple a plurality of seeds together end to end. However, the links couple to the seeds solely by an interference fit and, as such, are subject to failure upon the application of tensile force. Moreover, the spacers disclosed in Russel, Jr. are not bioabsorbable and remain intact in the human body, a feature which is not desirable once the seeds have been properly positioned at the treatment site.